The present invention pertains to methods and systems for stripping photo-resist material and removing etch-related residues from The Surface of a Partially Fabricated Integrated circuit in preparation for further processing. More specifically, the invention Pertains to Methods and Systems for Implementing a Plasma Operation that Includes introducing an inert gas downstream of the plasma source. The invention is effective at efficiently stripping photo-resist and removing residues from low-k dielectric layers after etching processes used to produce Damascene devices.
Damascene processing techniques are often preferred methods in modern integrated circuit manufacturing schemes because they require fewer processing steps and offers a higher yield than other methods. Damascene processing involves forming metal conductors on integrated circuits by forming inlaid metal lines in trenches and vias in a dielectric layer (inter-metal dielectric). As part of the Damascene process, a layer of photoresist is deposited on a dielectric layer. The photoresist is a light-sensitive organic polymer which can be “spun on” in liquid form and dries to a solid thin film. The photosensitive photoresist is then patterned using light through the mask and wet solvent. A plasma etching process (dry etch) is then used to etch exposed portions of dielectric and transfer the pattern into the dielectric, forming vias and trenches in the dielectric layer.
Once the dielectric layer is etched, the photoresist must be stripped and any etch-related residues must be thoroughly removed before subsequent processing to avoid embedding impurities in the device. Conventional processes for stripping photoresist employ a plasma formed from a mixture of gases with the presence of oxygen in the plasma. The highly reactive oxygen based plasma reacts with and oxidizes the organic photoresist to form volatile components that are carried away from the wafer surface.
Highly oxidizing conditions are also generally unsuitable for use on low dielectric constant (low-k) materials. Low-k materials have been used as inter-metal and/or inter-layer dielectrics between conductive interconnects employed to reduce the delay in signal propagation due to capacitive effects. The lower the dielectric constant of the dielectric material, the lower the capacitance of the dielectric and the lower the RC delay of the integrated circuit. Typically, low-k dielectrics are silicon-oxide based materials with some amount of incorporated carbon, commonly referred to as carbon doped oxide (CDO). It is believed, although not necessarily proven, that the oxygen scavenges or removes carbon from the low-k materials. In many of these materials such as CDOs, the presence of carbon is instrumental in providing a low dielectric constant. Hence, to the extent that the oxygen removes carbon from these materials, it effectively increases the dielectric constant. As processes used to fabricate integrated circuits move toward smaller and smaller dimensions and requires the use of dielectric materials having lower and lower dielectric constants, it has been found that the conventional strip plasma conditions are not suitable.
Hydrogen plasmas or hydrogen-based plasmas with a weak oxidizing agent are effective at stripping photo-resist and removing residues from low-k dielectric layers without the problems associated with conventional strip plasmas. However, these methods require a high hydrogen flow to achieve an acceptable strip rate. Because high hydrogen flow requires costly abatement and pump systems, it is desirable to have hydrogen flow as low as possible while maintaining an acceptable strip rate. In addition, it is desirable to reduce hydrogen flow due to hydrogen's flammability and the dangers associated with handling and abating it.
Others have reported using hydrogen-based plasmas with inert gases such as hydrogen and helium introduced with hydrogen at the plasma source. Han et al (U.S. Pat. Nos. 6,281,135 and 6,638,875) describe using a mixture of hydrogen, helium and fluorine and Zhao et al (U.S. Pat. Nos. 5,660,682 and 6,204,192) describe using a mixture of hydrogen and argon. However, helium or argon ions in the plasma have harmful effects. Mixtures of hydrogen and helium result in high plasma damage on low-k materials due to the long life of ionized helium plasma. Ionized argon causes unwanted sputtering of the quartz material in the plasma tube (the portion of some reactors where the plasma is formed). Introduction of argon to hydrogen plasmas has also been shown to reduce strip rate.
What is needed therefore are improved and methods and apparatus for stripping photoresist and etch-related materials from dielectric materials, especially from low-k dielectric materials, which reduce the required hydrogen flow rate while maintaining an acceptable strip rate.